I don't see why you cannot split the ring and add further length, but if that requires > 13 A (how much more?) that leaves very little for the rest of the sockets.

Perhaps the architects should have thought of the air-con in the first place?

I need 15 amps. Is this allowable in any reasonably achievable way please? In terms of the architects, the less said the better. The house has no ventilation, huge windows and needs an absolute tonne of AC installing because of poor material and design choices that have created an incredibly unusable house. It was 27 degrees in a bedroom the other day with no heating in use and an outdoor air temperature of 15 degrees!

Could you please specify which ring circuit you are looking to supply from? Are you referring to those servicing the kitchen area or the upper floor etc ? 

could be possible provided that, under the intended conditions of use, the load current in any part of the ring is unlikely to exceed for long periods the current carrying capacity of the cable as installed, assuming the cable has a installed rating of 20 amperes. 

Luckily the house has quite a few rings, it could be the rear top floor ring, or the middle floor rear ring. Both rings are just bedrooms and a study on the middle. 

Or as Chris suggested, split the ring over two 20 Amp radials. But again would need to check current carrying capacity of the cables as installed.

For a Ring Main, then there is no limit, probably 1000 amps is enough. Though you mean a ring final circuit, not a Ring Main. Presumably this will be a standard circuit with a 32 amp Type B circuit breaker with 2.5mm T+E cabling?

What is the load of the appliance, and will it run at that load all the time, or is that the start up current?

I'd be happy to fit an air con on an existing 32 amp circuit if I knew it would typically be running at 5 amps or less, with a 15 amp inrush to start the motor. Also, the existing appliances need to be taken into account. If there's already an oven, microwave and kettle on it, then I'd be reluctant to add to the circuit, but if, say, its feeding bedrooms, which have a maximum demand when the hoover is plugged in, then it may be fine to tap off that circuit.

How do you cope with start up/inrush currents? I know some Heat Pumps require a type C circuit breaker to account for the inrush, so does yours need a type C circuit breaker?  If so, will the existing circuits characteristics be able to satisy the requirements to be able to fit the tybe C?

It isnt easy, and no one can give you an answer without looking at the circumstances, it could be fine in the real world by tapping off an existing circuit, but it could cause numerous problems too, so I can see why your Electrician is not keen on using the existing circuit, as it will be him who is liable for the repairs/hassle/replacement if it does not work. To be fair, virtually every air con unit I've seen/fitted have their own circuit, so I can see his reluctance to fit it to your requirements.

Worth checking the MI (Manufacturers Instruction) as a lot of AC units need a didicated circuit with localised rotary isolation.  What is the max power consumption of the unit?  Could be 15 to 20 amps.  Also the breaker type recommended could be a type F or B to deal with the inrush current.  Remember MCB trip and RCBO mA trip are different characteristics.  Also factor in the volt drop calculations if use 2.5mm2 CSA and make note of any thermal derating as well as Earth leakage to not exceed 30mA.

If all good then 1 spur off a Ring Final Circuit COULD be OK.  its loaded with Caveats. 

The electrician is saying that 13a is the maximum, but I always thought that t was a little more. 

If you go back far enough (e.g. 15th Ed appendix 5) - the rule for for permanently connected equipment fed from a ring was a max 13A fuse or a max 16A circuit breaker - which is perhaps where your thought hails from. In later editions the rules for rings became less prescriptive (more rules of thumb than absolute regulations) and the 16A option wasn't mentioned any more.

There is more emphasis these days on ensuring that the (20A) cable of each leg isn't going to be seriously overloaded (whether by total load, or loads not well balanced around the ring, allowing the short leg to carry an overload while the longer leg remains underutilized) - perhaps including during unusual but foreseeable usage (e.g. how many fan heaters would likely be plugged in (and for how long) if the gas boiler went on the blink?).

So I'd suggest a 16A MCB is still an option - but as a designer the emphasis would be on you to show it would be satisfactory in that particular situation as you wouldn't be able to just rely on the traditional rules of thumb.

   - Andy.

Use 4mm² cable for the spur - equivalent to extending the ring with both legs in the same place.